This invention relates to DC to DC converters and, more particularly, to an improvement to a DC to DC converter in which the output voltage may be regulated against variations in input voltage and output current of the type described in U.S. Pat. No. 4,017,784. A DC to DC converter, as is known, converts the DC voltage provided by a DC source at an input to an output voltage level which may be different from that of the DC source. By way of example, this type of voltage-converting device finds application as part of traveling wave tube amplifier system where a low voltage DC source must be converted to one or more high voltage DC sources suitable to operate the electrodes of a traveling wave tube.
One converter which has been heretofore employed in such a traveling wave tube amplifier system uses a transformer having a primary and a center tapped secondary, with the secondary winding ends connected through rectifiers and connected in series with a large inductor for connection to one end of a load between the inductor and the secondary winding tap and with a pair of switches, such as transistor switches, connected in series circuit to alternate ends of the input, with one end of the primary winding connected to the juncture of the two transistor switches and the other end of the primary winding connected to the juncture of a series connected pair of capacitors connected across the input, with means responsive to the load voltage for varying the periodicity of control pulses alternately applied to the transistors. A sophisticated example of this type of circuit of which we have knowledge appears in U.S. Pat. No. 3,745,440 to Lord, owned by the assignee of the present invention. In the aforedescribed circuit, to an approximation, the current passed by the inverter transistors into the transformer primary is of a ramplike waveform; the current increases linearly in level with lapse of time to a predetermined maximum and then abruptly terminates. The output voltage is controlled by means varying the ratio of the "on" time of the transistor switches to the time of a half cycle. It is recognized that the inductor used in such device is bulky and heavy. Moreover, as a result of the ramp-shaped waveform of the primary current, the peak current through the transistor switches is at least twice the average current. This limits the peak power which can safely be handled by the device because of current and voltage limitations on presently available transistors. Additionally, the transistor must switch from a current-conducting or "on" state to a noncurrent-conducting or "off" state at a time when its collector current is at the highest level. Thus, in the foregoing converter a good deal of power is dissipated in the transistors during the interval in which the transistor is turning off. Furthermore, an undesirable side-effect of the fast rise and fall of current in the operation of the aforedescribed prior art circuit, the converter generates levels of high frequency electromagnetic energy that could cause interference.
By way of further background, a voltage converter design is presented in U.S. Pat. No. 3,582,754 to Hoffman et al. Hoffman discloses a DC to DC converter that is self-oscillatory, i.e. a secondary winding on the transformer is used to provide AC voltage feedback to the switching transistors, but which does not contain any separate control circuit to vary the width or the duty cycle of his primary currents. The converter in the Hoffman patent shows the use of a capacitor which in conjunction with an inductance produces half sinusoids of current in a transformer primary and in which the leakage reactance existing between the primary and secondary winding is used as the inductance. The Hoffman converter shows the use of a pair of series connected diodes to clamp the peak voltage excursion at one location, not directly across the capacitor, within the circuit as is brought out in the patent. And Hoffman further includes an additional inductance in series between the capacitance and the diodes having a reactance value substantially equal and opposite to the reactance of the capacitor to prevent lowering of the operating frequency in his inverter arrangement, which in symbolic appearance resembles the present invention. The Hoffman circuit is intrinsically nonregulating, and line voltage regulation is achieved by the use of "add-on" devices. As becomes apparent hereinafter, although the circuit of Hoffman contains features which are similar to the structure found in the present invention, the arrangement and cooperation of elements differs and achieves a different result.
By way of further background to our invention, reference is made to U.S. Pat. No. 4,017,784, granted Apr. 12, 1977 to P. Launderville and D. Simmons, a co-inventor of the present invention, the disclosure of which is by reference incorporated herein in its entirety. A DC to DC converter is there disclosed in which a sinusoid-shaped current flows through the primary of the transformer and the converter's output voltage is regulated by spacing the alternate polarity sinusoidal currents more closely together or farther apart in time of occurrence as well as other structural and functional aspects which the interested reader may obtain from review of the cited patent. That DC to DC converter combination includes a transformer containing a primary winding and a secondary winding on a structure of magnetic material with said secondary winding being "loosely coupled" to the primary winding to provide a predetermined effective leakage inductance characteristic as reflected to said primary winding; rectifier means connected to said secondary winding for rectifying the AC from the secondary winding to DC; filter capacitor means coupled to the output of said rectifier means for smoothing said rectified voltage; means for connecting an electrical load across said filter capacitor means; a second capacitor, said second capacitor connected in series circuit with said primary winding; input means for receiving a DC voltage for conversion to a different DC voltage required by the electrical load; electronic switching means for periodically and alternately charging said second capacitor in a first direction over a first interval of time T followed by charging said second capacitor means in a second opposite direction over a second interval of time T in a charging current path going through said primary winding, said charging current being derived from a DC voltage applied at said input means, with the second capacitor being sized in its capacitance value relative to said transformer leakage inductance characteristic and to the electrical impedance characteristics of said electrical load and said filter capacitor means for causing said charging current in said current path to be of a waveform essentially of half sinusoids over a portion of each said time interval; clamping diode means coupled to said second capacitor means for limiting the level of voltage across said second capacitor means to below the level applied across said input means; and regulating means coupled to said electronic switching means for regulating said interval of time, T, or said periodicity thereof, as a function of voltage level monitored across said filter capacitor means for maintaining the voltage derived across said filter capacitor means through the inverter action at a constant level irrespective of DC voltage level variation at said input means.
As those skilled in the art appreciate, many factors limit the power handling capability, the output wattage of a given converter, including that disclosed in U.S. Pat. No. 4,017,784. A major factor is the "peak" current handling capability of the inverter transistors. Thus, if a given "peak" current in a circuit exceeds that permissible in a given inverter transistor, the expensive transistor will be "wiped-out" or destroyed, even though the "average" current through the transistor has not been exceeded, as is recognized by those skilled in the art. Therefore, the designer must ensure that the peak currents do not exceed the permissible level for a given transistor under foreseeable circuit operating conditions.
A somewhat direct relationship exists between the peak and average currents in the inverter transistor circuit: By reducing the peak current allowed to pass through the transistor the average current through the transistor is also reduced. And since the power output of the transistor is measured in terms of the average current, the power output is accordingly also reduced. Frequently this reduction, based on the foregoing design considerations, places the actual converter power output below the actual power handling capability of the converter's inverter transistors.
One condition in which large peak currents occur, common to many electronic circuits, is when the converter is turned "on" and operating electrical power is initially applied to the converter. The large filter capacitor located in the secondary circuit of the converter is initially uncharged and at that time presents a very low electrical impedance at the secondary winding; hence, the charging currents are very large until the secondary capacitor becomes charged in normal converter operation. That impedance is reflected into the primary winding, as is understood by those skilled in the art, and during the initial period the inverter transistors must carry electrical currents much larger than they pass when the secondary filter capacitor is charged and the converter is operating in its "steady-state" condition.
The present invention is based upon our discovery that an electrical inductance means placed in series circuit between said primary or "first" capacitor and the diodes in the previously described converter of U.S. Pat. No. 4,017,784, provides beneficial results in relation to peak currents. If the inductance means is of a certain inductance value or size, the "peak" electrical current through the inverter switching transistors, at the time of initial turn-on or activation, is reduced. And if the inductance means is of a second inductance value the "peak" electrical current through the inverter transistor switches, during steady-state operation of the converter, is also reduced.
A principal purpose of our invention therefore is to provide an improved DC to DC converter and particularly to provide an improvement to the DC to DC converter of the type disclosed in U.S. Pat. No. 4,017,784 having increased power output capability. An ancillary object is to reduce peak currents carried by inverter transistors without significantly reducing average current carried by those transistors. An overall object of our invention is to provide a more reliable DC to DC converter.